The present disclosure relates to the subject matter disclosed in German applications No. 100 65 192.5 of Dec. 18, 2000 and No. 101 10 102.3 of Mar. 2, 2001, which are incorporated herein by reference in their entirety and for all purposes.
The invention relates to a rotor blade of hybrid design comprising a vane and a blade root, the vane comprising a first vane portion made of a metallic material and a second vane portion made of a lightweight material.
Such a rotor blade is described in DE 197 51 129 C1. Herein the vane is held in a slot formed on a metallic vane portion.
A rotor blade is known from EP 0 764 763 A1, wherein a second segment made of a composite material is placed in a first segment made of a metallic material.
In the article xe2x80x9cEin starker Verdichterxe2x80x9d (xe2x80x9cA strong compressorxe2x80x9d) in the DLR-Nachrichten (DLR News) of June 2000, pages 54-57, a hybrid blade comprising a part made from a carbon fiber composite material is described.
Further rotor blades of hybrid design are known, for example, from DE 1 628 355, GB 2 264 755 A, U.S. Pat. No. 3 883 267, DE 195 35 713 A1, DE 26 31 856 C2 or DE 2 042 665.
Starting therefrom, the object underlying the invention is to create a rotor blade of hybrid design having, on the one hand, a low mass, and, on the other hand, a high bearing capacity with respect to loads.
This object is accomplished with the rotor blade mentioned at the outset, in accordance with the invention, in that a rear edge of the rotor blade is formed on the second vane portion, and in that the second vane portion is joined to the first vane portion in a single-section-type manner.
With the hybrid construction, various materials with different physical properties are combined with one another to achieve an optimum design for a rotor blade. A rear edge of the rotor blade has essentially only an aerodynamic function and can, therefore, be formed on the second vane portion made of the lightweight material. The weight of the rotor blade according to the invention can be minimized by such a xe2x80x9clightxe2x80x9d rear edge.
When joining two parts made of different materials, mechanical stresses are caused, in principle, in transition areas, on the one hand, by the manufacture, and, on the other hand, by loads during use, which are due to different material behavior and to different physical and chemical material properties, such as, for example, thermal expansion, shrinkage processes during polymerization and curing (hardening) and different rigidities and different transverse contraction behavior. Hydrostatic stresses between the first vane portion and the second vane portion are strongly reduced by the second vane portion being joined to the first vane portion in a single-section-type manner. Such stresses occur when one part is at least partially enclosed and/or embraced by a second part, and three-dimensional constraints are caused thereby. These occur, in particular, when two parts embrace one another in a clamp-like fashion. Owing to the single-section-type joint according to the invention, deviatoric stresses preferably occur during the manufacture, and, in contrast to hydrostatic stresses, these can already be relaxed by flow processes in the course of manufacture of the single-section-type joint, with the second vane portion joined to the first vane portion in a tapered manner with a gradual increase in the width of one portion and a corresponding gradual decrease in the width of the other portion. Fewer material problems thus occur with the rotor blade according to the invention, as an homogeneous transition is achieved between the first vane portion and the second vane portion. At the same time, however, mass can be saved, which, in turn, results in a reduction of the load on the blade root.
The combination of a light rear edge with a single-section-type joint thus results in the mass of the rotor blade being reduced and in material problems, such as occur with the hybrid design, being at least diminished.
During operation, a rotor blade may be subjected to high temperatures, which results in a corresponding thermal expansion of the material of the rotor blade. With three-dimensional constraints, this can lead to the occurrence of stresses which limit the stability of the rotor blade under load. The danger of breakage caused by thermal stresses is strongly reduced by the single-section-type joint according to the invention, with which three-dimensional constraints are essentially avoided, because these stresses are easier to reduce or do not occur or not to the same extent as when three-dimensional constraints prevail.
In accordance with the invention, a rotor blade with a high bearing capacity with respect to dynamic and quasistatic loads and also with respect to impact loads such as bird strikes and with a relatively low mass can then be created.
It is of advantage for the second vane portion to be joined to the first vane portion in a tapered manner with a gradual increase in the width of one portion and a corresponding gradual decrease in the width of the other portion, so as to avoid material problems in the transition area between the first vane portion and the second vane portion.
It is of particular advantage for a front edge of the rotor blade to be formed on the first vane portion (xe2x80x9cheavy front edgexe2x80x9d). A stagnation point of the air stream flowing around the rotor blade lies at the front edge. The latter is, therefore, under heavy load, and the metallic material of the first vane portion ensures good protection against erosion. Moreover, the danger of bird impact is also considerably higher in the area of the front edge of the rotor blade. The metallic material of the first vane portion can absorb the energy of the bird impact irreversibly by plastic deformation without loss of the vane itself. This means that the operation of an engine equipped with rotor blades according to the invention is still possible in spite of bird impact.
The first vane portion advantageously protrudes in a front portion thereof beyond the second vane portion. An increased resistance to erosion with respect to the front edge is thereby achieved, and, on the other hand, the total mass of the vane can be minimized. To this end, a front edge of the rotor blade is expediently formed in the front portion.
It is also particularly expedient for the second vane portion to protrude in a rear portion thereof beyond the first vane portion. In this way, the rear edge can be formed on the second vane portion, and the mass of the vane can thereby be further reduced and the load on the blade root thus decreased. The rear portion is designed so that it has essentially only an aerodynamic function, and its force load, in particular, with respect to centrifugal forces, buoyancy forces, erosion and impact such as by bird strikes is low or has only a slight probability of occurrence. In particular, the rear edge of the rotor blade is formed in the rear portion.
In order to form a single-section-type joint surface between the vane portions, with the vane portions joined together in a tapered manner with a gradual increase in the width of one portion and a corresponding gradual decrease in the width of the other portion, it is particularly expedient for the second vane portion to be arranged on the first vane portion. In addition, the rotor blade according to the invention can thereby be manufactured in a simple way, as the first vane portion can be manufactured separately from the second vane portion.
It is of advantage for the first vane portion to form a concave side of the rotor blade at a front edge of the rotor blade. Furthermore, it is expedient for the first vane portion to form a convex side of the rotor blade at a front edge of the rotor blade. The front edge is thus made of the metallic material and, in particular, a good resistance to erosion and impact is thus achieved.
It is also expedient for the second vane portion to form a concave side of the rotor blade at a rear edge of the rotor blade. In addition, the second vane portion forms a convex side of the rotor blade at a rear edge of the rotor blade. The total mass of the rotor blade is thereby reducible.
It is of particular advantage for the second vane portion to form an area of the rotor blade which has essentially only aerodynamic functions. The material of the second vane portion has, on the one hand, a lower density than the material of the first vane portion so as to reduce the mass of the rotor blade. On the other hand, however, it may under certain circumstances have a worse plastic behavior and is then more susceptible to brittle fractures, in particular, when it is a ceramic material. The force load should, therefore, be low in this area. It is possible to determine on a rotor blade areas which have a low force load and areas which have a high force load. Such a last-mentioned area is, in particular, the blade root area. If the areas with essentially only aerodynamic functions are made from the material of the second vane portion, an overall optimization of the rotor blade is then achieved.
It is expedient for the second vane portion and the first vane portion to be joined together in a joint surface formed at an area of the first vane portion and at an area of the second vane portion. A gradual transition (graduation) is thereby achieved between the materials, and hydrostatic stresses are thus avoided in a simple way, and, depending on the material, deviatoric stresses are already reducible during the manufacture by the possibility of plastic flow, and, as a result of this, the danger of material flow during operation is avoided or at least reduced.
In order to achieve a single-section joint between the two vane portions, with the vane portions joined together in a tapered manner with a gradual increase in the width of one portion and a corresponding gradual decrease in the width of the other portion, the joint surface should be designed such that a curvature radius is substantially larger than the lateral extent of the joint surface, and, in particular, at least five times, and, advantageously, at least ten times larger than this lateral extent. In the case of a flat joint surface, the curvature radius is infinite.
An alternative or additional criterion for design of a joint surface in a tapered manner with a gradual increase in the width of one vane portion and a corresponding gradual decrease in the width of the other vane portion, is that the deviation of normal vectors of the joint surface from an average normal vector of the joint surface be at the most 20xc2x0 and, in particular, be less than 10xc2x0. It is thereby ensured that the joint surface is not of double-section design. This serves to avoid hydrostatic stresses in the joint surface.
A particularly suitable criterion for the presence of a single-section-type joint, with the vane portions joined together in a tapered manner with a gradual increase in the width of one portion and a corresponding gradual decrease in the width of the other portion, is when a curve formed by the section of the joint surface with a profile section is in one-to-one correspondence with the median line of the profile section. A median line of a profile is defined as the connecting curve of the central points of the circles inscribed in the profile and touching the latter at two points. The median line of the profile section is then the central point curve of the circles inscribed in the two-dimensional profile section. If the relation of the intersection curve of the joint surface with the profile section relative to the median line of the profile section along its arc length is in one-to-one correspondence, this correlation can then be represented by a stringently monotonic function. For example, a corresponding functional value along the arc line is obtained by the location of the intersection curve, normalized to the local width at the median line, being drawn on the normal to the median line within a profile section. Only if the resulting course of the function is in one-to-one correspondence (stringently monotonic), is there a single-section-type joint, with the vane portions joined together in a tapered manner with a gradual increase in the width of one portion and a corresponding gradual decrease in the width of the other portion.
It is of advantage for the blade root to be joined to the first vane portion. The force load of the rotor blade at the blade root is at its highest, as, in particular, the centrifugal forces and possible force loads owing to impact are highest there. Owing to the blade root being joined to the first vane portion, and, in particular, being integrally formed thereon, it is ensured that the ductile isotropic metal can absorb the forces without the occurrence of brittle cracks which, in turn, can result in brittle fracture.
It is of advantage for the second vane portion to be arranged outside an adjoining area of the blade root. Increased force load also prevails around an adjoining area of the blade root. The second vane portion with the brittle material and the light rear edge in comparison with the heavier front edge is then sufficiently far removed from regions of increased force load.
It is expedient for the rotor blade to comprise a first area between a convex side and a concave side of the rotor blade, with the first area being made of the metallic material, a second area between the convex side and the concave side of the rotor blade, with the second area being made of the lightweight material, and a transition area between the convex side and the concave side of the rotor blade, with the transition area partially including the metallic material and partially including the lightweight material. There is an homogeneous transition (graduation) between the materials in this transition area, and, therefore, material problems are reduced there. In particular, it is advantageous for a gradual transition of the materials with respect to the distance between convex side and concave side of the rotor blade to occur in the transition area, in order, on the one hand, to reduce the material problems in the transition area and, on the other hand, to improve the joint and avoid hydrostatic stresses.
It is expedient for the transition area to be approximately linear in a cross-sectional area of the rotor blade. The linear transition is only to be understood as first approximation. With such a linear transition, it is guaranteed that the joint surface will be of single-section-type, with the second vane portion being joined to the first vane portion in a tapered manner with a gradual increase in the width of one vane portion and a corresponding gradual decrease in the width of the other vane portion, and hydrostatic stresses are then substantially avoided.
It may, however, be provided that the transition area deviates from the linearity in that the second vane portion is enlarged towards the concave side of the rotor blade in the transition area. The second vane portion then preferably expands at the end of the transition area facing the rear edge to a greater extent than linearly, i.e., it bulges outwardly into the first vane portion. The compatability of the different materials of the two vane portions is thereby increased: With a metallic material as used in the first vane portion, the shear deformability is, as a rule, negligible, whereas the material of the second vane portion is soft with respect to a transverse deformability. Owing to the bulge extending into the metal, it is ensured that the latter will taper off thinly, whereby the different deformation patterns of the materials are adapted to each other by the geometrical configuration of the transition area. It is preferable for the first vane portion to taper off in a thin foil.
It may also be provided that the first vane portion is enlarged towards the convex side in the transition area, i.e., the metallic material has a bulge extending in the direction towards the convex side of the rotor blade into the second vane portion. In particular, the bulge of the metallic material lies in that area of the transition area which is closer to the front edge. Stress peaks are thereby avoidable in this area, as the geometrical configuration of the joint surface ensures a xe2x80x9cflatterxe2x80x9d tapering-off. The susceptibility to start cracking is thereby reduced in this area.
In a variant of an embodiment, it is provided that in the transition area closer to the front edge the first vane portion is enlarged with respect to a linear course, and in the area of the transition area closer to the rear edge, the second vane portion is enlarged with respect to a linear course. The metallic material then extends in a bulge in the transition area facing the convex side, and the lightweight material extends in a bulge in the transition area facing the concave side of the rotor blade. In this variant of an embodiment, an intersection curve between the joint surface and a profile section then has a turning point with respect to a straight line. In this variant, the transition area is geometrically configured such that in the area lying closer to the front edge, the susceptibility to start cracking is reduced, and in the area lying closer to the rear edge an improved compatability of the different materials with respect to shear deformations is achieved.
To optimize a rotor blade according to the invention with respect to stability under load, on the one hand, and mass, on the other hand, it is expedient for the first vane portion to laterally occupy approximately 20% to 45% of the convex side of the rotor blade at least in an area remote from the blade root. It is further expedient for the first vane portion to laterally occupy 50% to 75% of a concave side of the rotor blade at least in an area remote from the blade root. In particular, the first vane portion should occupy approximately ⅓ of the convex side and approximately ⅔ of the concave side. This results in an optimum transition area, by means of which, on the one hand, a good joint (adhesion) is achievable between the two vane portions, and, on the other hand, a good reduction in mass is obtainable.
Titanium or a titanium compound has proven particularly advantageous as material for the first vane portion. A reinforcement of the basic metal material with silicon carbide fibers can also be provided. These materials are then referred to as titanium matrix composite (TMC). These have excellent strength and rigidity values.
It is of advantage for the material for the second vane portion to be a fiber composite. Such materials have a low density, i.e., are very light. With a rotor blade designed in accordance with the invention, the average load on the blade root can be reduced by, for example, 20%.
A particularly light fiber composite suitable as material for the second vane portion is a carbon fiber composite (CFK).
To enable secure holding of the second vane portion on the first vane portion, these are adhesively bonded to each other. In principle, it is conceivable to provide an adhesive material for this purpose. It can, however, also be provided that the bonding is achievable without a separate adhesive material by the application of a fiber layup or prepreg material to a correspondingly pretreated metal. If, in particular, the second vane portion is formed by a fiber layup or prepreg material which is placed on the first vane portion and then further processed, a good and homogeneous joint is thereby achieved. In the case of a fiber layup the further processing is in the form of resin impregnation within a mold. In the case of a prepreg material the further processing is effected by consolidation and, in particular, pressing.
In order to achieve a good bonding, it is advantageous for the first vane portion to have a roughened surface in an overlapping area. An adhesive-matrix unit can thereby be formed at the joint surface, by means of which the second vane portion is adhesively joined to the first vane portion without an additional adhesive material having to be provided.
To achieve a secure bond between the two vane portions, it can be provided that the joint is reinforced by sewing or riveting in addition to the adhesion.
The invention further relates to a process for the manufacture of a rotor blade as described above.
The object underlying the invention is to provide a process which allows manufacture of a rotor blade which is optimized with respect to loading and mass.
This object is accomplished with the aforesaid process in accordance with the invention in that the first vane portion is manufactured, in that a surface area of the first vane portion is machined in preparation for the application of a lightweight material for formation of the second vane portion, and in that the lightweight material is then applied and a second vane portion formed.
One thus proceeds from a preformed first vane portion and then forms the second vane portion thereon. Thus, for example, the first vane portion can be used as part of a mold, and, in addition, a good joining of the joint is ensured.
In order to obtain a high-quality joining of the joint it is advantageous for the surface area to be cleaned. In particular, the latter is made grease-free by using, for example, solvents.
It is also advantageous for the surface area to be roughened. A good bonding between composite with its matrix structure and the metal surface is thereby achieved. It can, for example, be provided that the surface area is sandblasted for mechanical roughening. It can also be provided that the surface area is machined ultrasonically.
In a variant of an embodiment wherein, in particular, a thermosetting material is used as lightweight material, a fiber material is placed in a mold and infiltrated with resin which then cures. Alternatively, a prepreg lightweight material comprising preimpregnated fabric layers with fibers in a matrix can be used.
It is then expedient for a layup to be placed on the surface area and then further processed, in order to manufacture the second vane portion, on the one hand, and, at the same time, to bring about the joining with the first vane portion, on the other hand. The layup can be a fiber layup in which the matrix structure is produced by subsequent resin impregnation. It may also be a prepreg material which already comprises a matrix structure. If the fiber layup is placed in a mold and subsequently impregnated with resin, the specified shape of the rotor blade can be simultaneously produced and, on the other hand, the joining can be brought about. The joining can also be effected by the lightweight material being applied by means of a prepreg layup and after the application, consolidated and, in particular, pressed in a mold.
The ensuing description of a preferred embodiment serves in conjunction with the drawings to explain the invention in greater detail.